Once again, Beatriz Haddad Maia makes a good start to a service game but then gifts Sonay Kartal a break point with three consecutive unforced errors.

Kartal gets a look at a second serve but sends her return a fraction long.

A big roar from the Brazilian as she drills a cross-court backhand for a winner, but Kartal takes control of the next point with some powerful groundstrokes.

Another deuce to come...

Marta Kostyuk takes another convincing hold and restores that two-game cushion.

She still has that break advantage under her belt. Katie Boulter is going to have to make her move pretty quickly as this one begins to slip away from her.

Image source, Getty Images

A hold to love for Iga Swiatek piles the pressure on her opponent.

Emiliana Arango responds though and sees out her service game with an ace.

It's not all over just yet.

Better serving from Katie Boulter and she takes the hold to love. She even nailed a second serve in there.

Better from the Brit.

Image source, Getty Images

We're four games into set two and Marta Kostyuk gets the first hold on the board.

The cameras pan to show a service dog in the crowd. Finally, that's what we all want to see.

Sonay Kartal keeps the momentum going with a routine hold to move 4-1 up in the second set.

Leon Smith
GB Davis Cup captain on BBC Radio 5 Sports Extra 2 and BBC Sounds

When you get a break back it is important to cement that to get some momentum going, but Katie Boulter hasn't really been able to get it going at all in this match.

She is still hitting double faults and it doesn't look like she has got the feeling of the ball. The lack of confidence and match play is affecting her here.

That was unusual.

Katie Boulter's shot hits the top of the net pole and ends up dropping inside the lines on Marta Kostyuk's side of the court. She thought she had won the point but instead quickly returns to make sure of it.

Strange one so the umpire suggests replaying the point.

Iga Swiatek yet again demonstrates her powers of recovery.

Emiliana Arango was 30-0 ahead in the opening game of the second set but a double-fault at 30-30 gives Swiatek a third break point.

Arango shows aggression to save the first one, but Swiatek wins the next two points and secures an early break.

Could this be all over very quickly?

Image source, Getty Images

After losing the first set on a tie-break, 10th seed Lorenzo Musetti hits back early in the second with a break of serve against Giovanni Mpetshi-Perricard.

The big-serving Frenchman, who is seeking his first ever win at the US Open, double-faults to give Musetti the first break point of the match.

And a poor forehand volley from Mpetshi-Perricard then gifts the Italian an early advantage in the second set.

There's a healthy contingent of Brazilian supporters following Beatriz Haddad Maia on Court 12 and they're happy to see their player move 40-0 up in her latest service game.

But...

Her fourth double fault of the match and two errors peg her back to deuce.

She eventually wraps up the game but it was so much more difficult than it needed to be. In a match that looks poised to go into a deciding set, every ounce of energy might end up counting.

A loose forehand from Katie Boulter hands Marta Kostyuk another break of serve.

Neither of these two able to hold on to their serve in set two so far.

Marta Kostyuk takes the first two points off the Katie Boulter serve and it looks like neither of these are going to be able to hold serve in this second set.

But the Brit produces a gutsy serve down the T to level at 30-30 but an unconvincing move into the net takes us to deuce and a double fault hands Kostyuk a break point.

Image source, Getty Images

Sensational tennis from Iga Swiatek.

Emiliana Arango was 40-0 up on her own serve but the world number two regains control and secures the double break at the first opportunity.

Serving for the set, Swiatek is able to hold despite her third double fault of the match, with a superb backhand up the line ending the first set in just 26 minutes.

Leon Smith
GB Davis Cup captain on BBC Radio 5 Sports Extra 2 and BBC Sounds

For the first time we've seen Haddad Maia tighten up. She's hit a really slow serve, she must be feeling it a bit in this second set.

Sonay's dictating the play now.

That's unfortunate for Katie Boulter. Marta Kostyuk wipes out both break points as Boulter hits into the net for deuce.

The Brit will have a third opportunity as Kostyuk nudges into the net and the 27th seed hits a double fault to hand over the game.

Back to square one.

Much better from Sonay Kartal.

The Briton, ranked 51st in the world, wraps up the hold for a 3-0 lead in the second set after saving a break point earlier in the game.

Leon Smith
GB Davis Cup captain on BBC Radio 5 Sports Extra 2 and BBC Sounds

Katie Boulter needs to remember that her opponent has withdrawn in the last two tournaments. If I was her coaching team right now, I would be telling her to hang tough.

Marta Kostyuk looks OK right now but the longer Katie Boulter can keep this match going, you just never know what might happen. The first two or three games of this second set are going to be key.

An ace and a forehand winner drag Sonay Kartal back to 30-30 from 0-30 down in her latest service game, but then the Briton slices a backhand wide.

Break point for Beatriz Haddad Maia - but the Brazilian nets a drop shot. Deuce.

Bit of a wobble for Marta Kostyuk on serve this time but she gets back on track with a brilliant ace down the T.

The Ukrainian does well to reach a forehand but is on the backfoot and Katie Boulter takes advantage, forcing the 27th seed long to earn herself two break back points.

Image source, Getty Images